In general, polyesters such as polyethylene terephthalate are known to possess a good chemical stability, physical and mechanical strength, heat resistance, chemical resistance, weather resistance and electrical insulation property; and, therefore, have been widely used in manufacturing various articles including condensers, photographic films, packaging and labelling materials, and magnetic recording media. Particularly, in order for the polyester film to be suitable for use in preparing the magnetic recording media, it is required to have excellent runnability, scratch resistance and abrasion resistance.
In general, to prepare such a recording media quality polyester film, inorganic slip agents such as calcium carbonate, silica and alumina have been incorporated into the polyester film to form minute protuberances on the surface thereof, thereby reducing the area of contact between the film surface and, e.g., guide rolls in a video cassette recorder.
Among these inorganic slip agents, calcium carbonate of calcite structure are known to be capable of controlling surface properties of the film; however, they have a low affinity to polyester and low hardness, which may lead to the formation of voids during a drawing process of the film. In a further processing of the film, such voids may cause abrasion of the surface, separation of the particles and scratching of the film.
To ameliorate such problems, it has been proposed to employ alumina or silica mixed with a calcium carbonate of calcite structure as the slip agent. For instance, Japanese Patent Laid-open Publication No. Hei 2-214734 discloses a method for improving surface properties and abrasion resistance of a polyester film by incorporating .alpha.-, .gamma.-or .delta.-alumina and calcium carbonate of calcite structure into the polyester film.
Similarly, Japanese Patent Laid-open Publication No. Sho 64-48836 offers a polyester film having an improved scratch resistance, in which inert particles such as colloidal silica(A) and synthetic calcium carbonate(B) having a diameter not less than that of (A) are incorporated and the height of the protuberances on the surface thereof due to (A) is lower than that attributable to (B).
Recently, certain organic particle having a good affinity to polyethylene terephthalate, a mixture of two or more inorganic particles, or a mixture of organic and inorganic particles have been employed to control the shape of the protuberances on the surface of the film to obtain improved surface properties. For instance, U.S. Pat. No. 4,761,327 discloses a method for improving scratch resistance of a film by incorporating therein organic particles such as silicon together with an inorganic slip agent, thereby increasing the affinity of the inorganic particles to polyethylene terephthalate.
However, even though these prior art methods may improve surface properties, scratch resistance and abrasion resistance of a polyester film to a certain degree, there has still existed a need for a polyester film with more enhanced physical and chemical properties.